Implement quicksort in C

Implement **quicksort** in C, placing the code in `Sorting/` directory

Start by inspecting the existing files in the `Sorting/` directory and the repository’s other algorithm implementations to understand naming and layout conventions. Add the quicksort implementation in C there, and consider the work complete when it correctly sorts representative input cases and matches the surrounding project’s conventions.

Written by the indexing model from the issue text.